Self-phase modulation (SPM) is a phenomenon that happens when digital signals are transmitted through the optical fiber, which can be anything from signals to data from elearners or other web-based data coming through a fiber-optic line. It is a result of changing the optical fiber reflection according to the loaded power. SPM leads to undesired phase modulation of pulse fronts. As a result, the pulse is expanded, the interference between the symbols increases and the transmission speed decreases.
Cross-phase modulation (XPM) mainly happens when WDM signals are transmitted over the optical channel. It is similar to the SPM and dues to the signal interaction between two optical channels. When more than one signal is transmitted through the optical fiber each of them changes the reflect coefficient keeping the law of changing of the optical power. Investigations show that XPM decreases when the fiber effective area is increased.
Four wave mixing (FWM) is an event that is typical for DWDM systems. It is reason for creating products with new frequencies, and some of the products may fall into the used channels. There are two main factors which affect the level of FWM products, i.e. the mixing efficiency – channel spacing and the dispersion of the chosen optical fiber.